Because you are picking an arbitrary example, one that suits your argument. There could be effects like, "Gain 5% mana back from your heals." Cherry picking the more boring example does not really prove your point.
Even with preset builds, people choose to tailor their playing experience around the flavors of choice they like. For example, the same argument you made has always been used against talents in wow. Wow talents give you a bit more choice because most of the choices are minor. In most instances, it's only the final few talents that sort of force your hand with what builds you are using.
Also, why is the illusion of choice toxic to MMOs? That seems like a pretty brash statement without backing it up.
Also, we already have gear effects and set bonuses, go check your GC vendor before you assume we need some game overhaul to add gear depth.
We only need to turn our eyes to WoW to see that gear sets and horizontal progression don't cause toxicity, it does not cause the sky to fall, and the last I checked their servers did not spontaneously combust. People like set bonuses. People like weapons with unique effects. It has always been well received in other MMO's.
There, I provided you a game where gear depth accomplishes a lot of positives in respect to game play and player motivation. Where is your example of an MMO tragically hurt by more interesting gear options?
QFTx10000
The only rewards in this game are vanity. You only need to wait for the latest savage patch, buy the latest crafted gear, meld it and your effectively close to the ilvl cap. The gear treadmill in this game is already in such a state that content like Niddhog EX goes live with essentially vanity rewards.
If SE did horizontal progression like WoW, I would love it. Imagine if the niddhog weapons, being 235, actually had weapon effects and in the right hands those effects would make the weapons effectively ilvl 240/245 performance wise. That would be amazing.